1. Field of the Invention
The present invention relates to an optical element that uses a liquid crystal and a method for manufacturing the same and an optical apparatus and a method for manufacturing the same.
2. Description of the Related Art
Liquid crystal cells generally have a structure in which a liquid crystal is sandwiched between oriented films or between an oriented film and a transparent electrode and are capable of controlling phase of light or modulating it. The structure is, for example, such as is shown FIG. 11 through FIG. 13, that is, transparent electrodes 2 (ITO etc.) and oriented films 3 are provided on two sheets of glass substrates 1, the oriented films 3 are opposed with a constant spacing therebetween by the use of spacers 4, and a liquid crystal 5 is filled therein. Numeral 6 denotes a power supply.
When an external voltage is applied to this liquid cell, direction of orientation of liquid crystal molecules changes form a longitudinally arranged state shown in FIG. 11 (upward direction in the figure) to an obliquely arranged state shown in FIG. 12 (aslant upward direction in the figure). When the voltage is further increased, the liquid crystal finally reaches a transversely arranged state whose direction of arrangement is parallel to a direction of the applied voltage as shown in FIG. 13. If one controls the direction of orientation of liquid crystals in this way, direction of the index ellipsoid changes gradually from FIG. 14 to FIG. 16, which means that the refractive index which incident light senses varies, and therefore one can control phase of light or modulate it by using this variation of birefringence.
Normally, the oriented film consists of a polymer organic film provided on the substrate by spin coating and minute grooves are formed thereon in one direction by further rubbing the film with cloth. When no external voltage is applied on the liquid crystal cell, liquid crystal molecules are arranged along these grooves; when the external voltage is applied, the liquid crystal molecules are arranged to be gradually and continuously in parallel to a direction of the applied external voltage. However, since such a method of forming the oriented film involves a problem of light scattering, ununiformity, and inclusion of dusts, recently a method of forming the oriented film by means of exposure by ultraviolet light attracts public attention as its alternative method.
That is, this formation method, which is entirely different form the method in which grooves are formed physically as described above, aims to endow anisotropy to the oriented film (note: strictly speaking, any orientation film before being oriented should be called as an orientation film rather than an oriented film, but hereinafter term xe2x80x9coriented filmxe2x80x9d is used also for an orientation film for simplicity) by irradiating it with linearly polarized ultraviolet light, using photoisomerization reaction, photodimerization reaction, photodecomposition reaction, etc.
In most cases, macromolecules on the oriented film tends to arrange themselves in a direction normal to the direction of polarization of the ultraviolet light, and if the oriented film thus prepared is used, liquid crystals also arrange themselves in a direction normal to the direction of polarization of the ultraviolet light. Such a formation method has been developed to enlarge the angle of visibility in liquid displays as a main purpose (Published Unexamined Patent Applications No. Hei 10-90675 and No. Hei 10-123523) and is expected to find its applications in consumer electronic parts as well as liquid crystal displays.
On the other hand, magneto-optical apparatuses need separation of polarized lights in detecting signals, and a polarization hologram, in which a diffraction grating having polarization property was formed on LiNbO3 using proton ion exchange, has been proposed before to devise such a function (Published Unexamined Patent Application No. Hei 6-300921). However the fabrication method, in practice, has drawbacks of complexity and high fabrication cost.
Moreover, methods to correct coma aberrations occurred when an optical disk is driven and spherical aberration occurred when an optical disk whose substrate is of different thickness is read has been proposed (Published Unexamined Patent Applications No. Hei 10-20263 and No. Hei 10-92004) employing a liquid crystal element whose liquid crystal cell is divided into sections, each of which is provided with electrodes. However, weak points of this method are that wiring of transparent electrodes as well as a driving method are rather complex and that shielding of transmitted light and limited workability of electrodes due to electrode structure itself give rise to low efficiency and low accuracy of correction of wave front.
As for manufactures of holograms, a lot of methods are known besides the aforesaid methods. For example there is a method in which phase distribution for forming a desired diffraction pattern is calculated and then a hologram is manufactured based on the phase distribution. However, such methods as represented by the aforesaid method have common a problem, that is, most of elements used therein are static elements, and therefore it is difficult to manufacture a high-precision element having a dynamic function.
On the other hand, much attention has been paid to active materials of liquid crystals and now efforts aiming at dynamic diffractive optical elements using liquid crystals have established a method which provides such elements using liquid crystals (CLEO, Technical Digest, CTho34(1998)). However, this method involved problems, in a process that a liquid crystal is filled in on the diffraction grating, such as that the diffraction efficiency is dependent on the precision with which the original diffraction grating is fabricated and that the response speed of the liquid crystal becomes slower because a grating is substituted for the oriented film which is used for the liquid crystal, etc. Moreover, any hologram which uses a liquid crystal in fabrication, including those made by the aforesaid method, has not only a complicated structure but also a decreased grain size and degraded efficiency because area of the hologram is divided into sections and these sections are provided with respective electrodes on which different voltages are applied, as mentioned above.
By the way, there is other type of hologram proposed lately, in which polarization characteristic of a minute grating is used to modify the polarization state in order to realize amplitude modulation of light, but this method is also difficult to fabricate (xe2x80x9cDesign of diffractive optical elements modulating polarization, xe2x80x9d V. VKotlar, O. K. Zalyalof, Optik, Vol. 105, No.1, pp.1-6 (1997).
In addition to this, polarization plane rotation (i.e. rotation of the plane of polarization) becomes a big problem to be solved in optical apparatuses and optical elements such as polarizing microscopes, high numerical aperture objective lenses for the optical disk, etc. For example, an optical element called rectifier has been used to correct the polarization plane rotation caused by a high numerical aperture objective lens itself in the polarizing microscope. When the angle of incidence is large, the transmittance for linearly polarized light whose electric e field vector vibrates in a plane of incidence (p-polarization) and that for linearly polarized light whose electric field vector vibrates perpendicularly to the plane of incidence (s-polarization) do not coincide with each other. Therefore, when the incident light whose electric field vector forms an angle other than 0xc2x0 or 90xc2x0 to the plane of incidence (that is, when the light is neither only p-polarized light nor only s-polarized light), there inevitably occurs the polarization plane rotation. This polarization plane rotation can be corrected by the use of a combination of a half-wavelength plate and a null lens having zero refracting power.
Also, when light passes through a birefringent material or a medium in which residual birefringence occurs due to stress etc, the polarization state of the transmitted light is changed. These changes in the polarization state cause significant decrease in sensitivity of optical systems for detecting polarized light such as polarizing microscopes. Also, in a polarizing microscope having a high numerical aperture, light with a large angle of incidence exhibits a focused spot with a diffraction patter similar to that of astigmatic aberration in the plane of observation. This leads to degradation of the resolution and hence becomes a big problem together with degradation in sensitivity due to deterioration of the extinction ratio.
Similarly, in magneto-optical disks, large angle of incidence of light thereto causes the polarization plane rotation. Moreover, when a disk substrate has residual birefringence, reflected light suffers change of the polarization state. Furthermore, when a disk has a multi-layered structure such as those of magneto-optical disks etc., reflected light undergoes change of its phase because of interference of reflected light beams generated at respective layer boundaries, hence causing the change of the polarization state, and also the structure of grooves on the disk shows polarization-dependency due to diffraction.
In magneto-optical disks, the signal is reproduced by Kerr effect due to a magneto-optical effect of incident light, but typically the rotation angle of Kerr effect is as small as approximately 0.20xc2x0 to 0.50xc2x0. The signal is detected by separating polarized light in the magneto-optical signal light when detecting the signal, but light other than the light to be detected may filter in the signal light that is generated by the polarization plane rotation due to a high numerical aperture of the lens and a large angle of incidence of light to the disk. It is well considered that this component is almost a DC component, which varies sensitively to the output signal from a semiconductor laser, its focusing error and tracking error, etc. and hence causes deterioration of the S/N ratio of the signal. Therefore, any variation factors causing the change of the polarization state except that caused by the signal should be removed as much as possible. The polarization plane rotation was not so far a major problem in magneto-optic disks, but lately this has become a significant problem in conjunction with increasing numerical aperture of the objective lens.
The present invention has been devised in consideration of the situation described above and it is the object of the present invention to provide both an optical element capable of controlling the polarization state spatially by the use of a liquid crystal and an optical apparatus capable of correcting the polarization state in conformity to a particular purpose by the use of the liquid crystal as well as their manufactures.
The optical element according to the present invention comprises a liquid crystal element in which a liquid crystal is provided in contact with an oriented film, characterized in that the direction of orientation of liquid crystal molecules of the aforesaid liquid crystal are made to vary spatially (i.e. in a position-dependent manner), so that the polarization state of light is controlled spatially.
The optical apparatus according to the present invention is characterized in that the optical apparatus contains an optical element comprising a liquid crystal element which further comprises a liquid crystal disposed in contact with a oriented film, wherein the direction of orientation of liquid crystal molecules of the aforesaid liquid crystal are made to vary spatially so that the polarization state of light which is processed at the aforesaid optical element is controlled spatially and that the optical apparatus performs correction of polarized light by use of this optical element.
Method for manufacturing an optical element and method for manufacturing an optical apparatus according to the present invention are characterized in that, in manufacturing the optical element wherein the direction of orientation of liquid crystal molecules of the aforesaid liquid crystal caused by an oriented film is varied spatially and hence polarization of light processed (i.e. refracted or reflected) thereby is controlled spatially or in manufacturing an optical apparatus provided with the aforesaid optical element, each of the manufactures involves a step selected from a group consisting of: a step of performing light irradiation on an oriented film by using masks while the polarization state is being varied; and a step of performing light irradiation on an oriented film by using light irradiating means while the aforesaid oriented mask is being moved relative to the light irradiating means; and accordingly controllability of orientation that the direction of orientation of the aforesaid liquid crystal molecules is made to vary spatially is endowed to the aforesaid oriented film.
In this way, in the present invention the direction of orientation in the oriented film of the optical element comprising a liquid crystal is made to vary discretely or continuously, and thereby the direction of orientation of the liquid crystal is varied spatially (in a position dependent manner), so that polarization state of the incident light can be transformed to an arbitrary distribution of polarization states (arbitrary distribution of position-dependent polarization states).